The authors show a distal accumulation of lysosomes in aged neurons. These lysosomes appear less functional than in younger neurons and larger in size. This defect is associated with reduced synaptic activity. The study is well designed, the data are convincing and the subject fits very well within Traffic. However some points remain which could strengthen the authors conclusions

Open points:

1) Electron microscopy images of the lysosomes on DIV21 vs DIV28 in figure 1 might better show the increase in lysosomal size, as it is not visible in the confocal images, although significant at the quantification.
2) It would strengthen the data on lysosomal dysfunction and defect in trafficking if authors performed a transferrin uptake assay to complement the DQ-BSA.
3) “we were intrigued by the reduced lysosomal degradative capacity in the aged somas, given the increased lysosomal hydrolase, Cat D. Therefore, we used the fluorescent acidotropic probe, LysoTracker (Barral et al., 2022), to determine whether the acidification of ELs was optimal for degradative activity. “ If authors would like to make claims about proper acidification/ degradative capacity of lysosomes they should use Lysosensor rather than (or in addition to) lysotracker (in figure 1 3 and 4 for example), as the fluorescence of lysosensor is pH sensitive, and therefore changes in intensity will reflect changes in lysosomal pH. Lysotracker fluorescence is pH insensitive.
4) It would strengthen figure 7 to repeat at least some of the chloroquine experiments with Bafilomycin A1, as this specifically blocks the vATPase and therefore specifically lysosomal acidification, which the authors have emphasized several times as being affected in their phenotype.
5) The discussion should be reworked, there is a restatement of every single result, which is unnecessary. It would be better for authors to pull out only the key findings and put them into a larger context. For instance, the importance of lysosomes in the aged brain has also been shown in neural stem cells. Quiescent NSCs have more lysosomes than activated NSCs, and aged NSCs also have less functional lysosomes resulting in less differentiation. It would be interesting if authors could comment on their finding of neuronal lysosomal dysfunction in the context of lysosomal dysfunction in the aged brain as a whole. (Leeman, D.S., Hebestreit, K., Ruetz, T., Webb, A.E., McKay, A., Pollina, E.A., Dulken, B.W., Zhao, X., Yeo, R.W., Ho, T.T., et al. (2018). Lysosome activation clears aggregates and enhances quiescent neural stem cell activation during aging. Science 359, 1277–1283. )

Authors have clarified all the comments previously raised by this reviewer. Therefore i think it should be accepted for publication in Traffic.